Tar sands which are also known as oil sands and bituminous sands are siliceous materials which are impregnated with a heavy petroleum. The largest and most important deposits of the sands are the Athabasca sands, found in northern Alberta, Canada. These sands underlay more than 13,000 square miles at a depth of 0 to 2000 feet. Total recoverable reserves after extraction and processing are estimated at more than 300 billion barrels--just equal to the world-wide reserves of conventional oil, 60 percent of which is in the Middle East. By way of contrast, the American Petroleum Institute estimated total United States oil reserves at the end of 1965 at 39.4 billion barrels.
The tar sands are primarily silica, having closely associated therewith an oil film which varies from about 5 percent to 21 percent by weight, with a typical content of 13 weight percent of sand. The oil is quite viscous--6.degree. to 8.degree. API gravity--and contains typically 4.5 percent sulfur and 38 percent aromatics.
The sands contain, in addition to the oil and sand components, clay and silt in quantities of from 1 to 50 weight percent, more usually 10 to 30 percent. The sands also contain a small amount of water, in quantities of 1 to 10 percent by weight, in the form of a capsule around the sand grains.
Several basic extraction methods have been known for many years for the separation of oil from the sands. In the so called "cold water" method, the separation is accomplished by mixing the sands with a solvent capable of dissolving the bitumen constituent. The mixture is then introduced into a large volume of water, water with a surface agent added, or a solution of a neutral salt in water, which salt is capable of acting as an electrolyte. The combined mass is then subjected to a pressure or gravity separation.
In the hot water method, as disclosed in Canadian pat. No. 841,581 issued May 12, 1970 to Floyd et al., the bituminous sands are jetted with steam and mulled with a minor amount of hot water at temperatures of 170.degree. to 190.degree. F., and the resulting pulp is then dropped into a turbulent stream of circulating hot water and carried to a separation cell maintained at a temperature of about 185.degree. F. In the separation cell, sand settles to the bottom as tailings and oil rises to the top in the form of a froth. An aqueous middlings layer comprising clay and silt and about 1 to 5 weight percent bitumen based on the weight of the middlings is formed between these layers. This basic process may be combined with a scavenger step for further treatment of the middlings layer obtained from the primary separation step to recover an additional amount of oil therefrom.
The middlings layer withdrawn from the hot water separation cell in a hot water extraction process contains most of the silt and clay as well as some bitumen. In the hot water extraction process disclosed by Floyd et al. above, a stream of middlings is withdrawn from the primary extraction zone and recycled to the conditioning vessel of the hot water process. Also, a second stream of the middlings is transferred from the primary extraction zone to an air scavenger zone wherein air is bubbled into the material in the scavenger zone to cause flotation of additional bitumen from the middlings material. This bitumen is then recovered as a froth and combined with the bitumen froth recovered from the primary extraction zone. The depleted middlings stream which now contains less bitumen, usually 0.5 to 2.0 weight percent, is normally thereafter discarded into a retention pond or in some circumstances combined with the sand tailings layer which was removed from the primary extraction zone and subsequently discarded.
One of the problems encountered in the above described hot water process is the inefficiency of recovery which sometimes occurs in the primary extraction zone. After the viscosity of the middlings increases, poor flotation is relaized with resulting lower bitumen recovery rates.
The present invention provides an improvement to the above-disclosed hot water extraction process which aids in improving recovery of bitumen from tar sands in a primary extraction zone of a hot water process.
For purposes of definition, in the present disclosure the term "bitumen-rich middlings" defines a middlings material recovered from the gravity settling separation zone of a hot water process for extracting bitumen from tar sands which middlings material is characterized as containing water, silt, clay, and about 1.0 to 5.0 weight percent bitumen. In turn, the term "bitumen-lean middlings" or "bitumen-depleted middlings" defines bitumen-rich middlings which have been treated in an air scavenger zone to extract bitumen and, therefore, result in middlings containing 0.5 to 2.0 weight percent bitumen. In all events, bitumen-lean middlings always contain less bitumen than the bitumen-rich middlings material whence they came.